Photon Factory Activity Report 2018 #36 (2019)

BL-11D/2017G638

Fabrication of Cr/Sc/Mo multilayer illuminator for water window soft X-ray
microscopes with Bi plasma sources
Tadashi HATANO1,* Takeo EJIMA1, Yuichi ONO2, Thanh-Hung DINH3, Toshiki TAMURA2, Keigo
MAEDA2, Hiroyuki HARA2 and Takeshi HIGASHIGUCHI2
1 Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan
3National Institutes for Quantum and Radiological Science and Technology, 8-1-7 Umemidai,Kizugawa, Kyoto 619-0215, Japan
1 Introduction
Living cell observation in 10 nm order resolution canbe expected when the water window soft X-rays between
K-absorption edges of carbon and oxygen are used. To
realize that, a soft X-ray microscope composed of laserproduced plasma source, condenser mirror, scintillator,
and imaging lens optics is under construction. Unresolved
transition array (UTA) radiated from highly ionized highZ elements will be useful as a high power source inlaboratory scale [1]. For the purpose of water window
microscope, UTA of Bi plasma around 310 eV with a10% bandwidth will be useful if a suitable condenser
mirror is available. In our microscope system, a toroidal
mirror of a large effective size with an angle of incidenceof 77.2° which is much smaller than the critical angle, isused for high flux illumination with plasma soft X-ray
sources of a large angular divergence. The mirror iscoated with a multilayer for a specific wavelength region
or coated with Au for general use. In our previous study, aCo/C multilayer mirror was developed for 270 eV carbon
window soft X-ray [2]. Recently we found a multilayer
structure of Cr/Sc/Mo 10 tri-layer produces a highreflectance and a 10% bandwidth around 310 eVmatching the UTA of Bi-plasma [3, 4]. In this study Cr/
Sc/Mo multilayers have been deposited on toroidal mirror
substrates with thickness distribution controlled forspectral matching.
2 Multilayer deposition
Dimensions of the toroidal mirror substrate areillustrated in fig. 1 [2]. Angle of incidence varies between
76.5° and 77.2° along the mirror surface in the meridional
plane. Every layer must have lateral thickness distribution
for spectral matching against change of angle ofincidence. Designed thickness distribution normalized by
the thickness at the center, d(w) is shown by a thick curvein fig. 2. On the other hand, each material has deposition
rate distribution depending on the geometry of thedeposition apparatus, in general. Deposition rates of Cr,
Sc and Mo in our ion beam sputtering apparatus, sCr(w),sSc(w) and sMo(w), respectively, were measured by X-ray
reflectometry on Cr/Sc, Sc/Mo and Mo/Cr multilayers.
The results are shown by thin curves in fig. 2. Decrease ins(w) toward w → ±50 mm is larger than decrease in d(w),which means the deposition time t(w) = d(w)/s(w) should
increase. Material dependent deposition time functions

relative thickness ordeposition rate

1.05
design thickness

10.95

ScCr

0.90.85

deposition rate of Mo

0.8-50 -40 -30 -20 -10 0 10 20 30 40 50position, w (mm)Fig. 1: Schematic drawing of the toroidal condenser
mirror substrate [2].

Fig. 2: Designed thickness distribution and deposition
rate distribution of Cr, Sc and Mo, normalized by thevalues at the center.

1.2

0.3

1.150.25
deposition time of Mo
reflectance

relative deposition time

Photon Factory Activity Report 2018 #36 (2019)

1.1
1.05

CrSc

10.2center(φ = 77.2°)
w = 20 mm(φ = 77.1°)

0.15
0.1
w = 40 mm(φ = 76.7°)

0.05

0.95

l = 10 mm(φ = 77.2°)

0-50 -40 -30 -20 -10 0 10 20 30 40 50

260

position, w (mm)

300

320

340

360

photon energy (eV)Fig. 3: Deposition time functions of Cr, Sc and Mo.
are shown in fig. 3, and our deposition apparatus withspeed controlled shuttering system can realize them [5].
Thickness distribution in the sagittal plane was notcontrolled.
Deposition started with Mo layer followed by 10 Cr/
Sc/Mo tri-layers and terminated with Cr layer.
Thicknesses of tri-layer are almost equivalent in first few
cycles and then transparent Sc and absorptive Mo became
thicker and thiner, respectively. Top layers are periodic Cr(3.4 nm)/Sc (4.8 nm)/Mo (1.7 nm).
3 Reflectance measurement
Reflectance measurements were performed at the soft
X-ray reflectometry beamline BL-11D at the Photon
Factory. Spectral reflectance around 310 eV weremeasured at the center of the substrate and other threepoints shown in fig. 4. Angle of incidences were set as thesame as the condition under condenser mirror use. Results
are shown in fig. 5. Peak reflectances were 18-22%.
Reflection bandwidth was 30 eV. Reflection peak energy
was 310 eV at every point except for the point out of themeridional plane. High reflectance and large bandwidth

42center
00.5
-0.05

0.00

280

0.00.05-0.5

Fig. 4: Measurement points on the toroidal mirror
surface.

Fig. 5: Measured spectral reflectance of Cr/Sc/Mo
multilayer.
with peak energy 310 eV matching the UTA of Bi-plasma
sources were confirmed
Acknowledgement
This work was partly supported by CAMS Project
2015, IMRAM, Tohoku University.
References[1] T. Higashiguchi et al., Appl. Phys. Lett. 100, 014103(2012).
[2] T. Ejima et al., Appl. Opt. 53, 6846 (2014).
[3] T. Hatano et al., J. Electron Spectrosc. Relat. Phenom.
220, 14 (2017).
[4] T. Hatano et al., Photon Factory Activity Report 2015#33, 351 (2016).
[5] T. Hatano et al., Photon Factory Activity Report 2011#29, 408 (2012).
* hatanotadashi@tohoku.ac.jp